1. Field of the Invention
This invention relates to the field of semiconductor photo detectors used in opto-couplers, and particularly to reducing the disabling effect of displacement current in photo diodes used in opto couplers.
2. Description of the Related Art
Opto couplers are used to allow two electrical systems to communicate with each other while remaining electrically isolated. This communication is achieved by sending light signals using an electrically activated light emitter, typically a Light Emitting Diode (LED), to a photo detector which converts the light signals back into electrical signals. The light passes through a transparent insulator thereby electrically isolating the light emitter and its associated circuitry from the light detector and its associated circuitry. The associated circuitry of the LED side of the optocoupler can include an LED driver, amplifier, etc. The associated detector circuitry can include an amplifier, output drivers, A to D and D to A converters, etc.
Unfortunately, a displacement current can flow from the LED side of the opto coupler into the photo detector and cause an electrical output from the photo detector in addition to the electrical output produced by the light. The displacement current is a consequence of the unavoidable capacitance coupling between the LED side of the opto-coupler and the photo detector. This displacement current flows when the voltage applied to the coupler's LED and its surrounding electrically conductive structures changes with respect to the voltage applied to the coupler's photo detector. The magnitude of the unwanted displacement current flowing from the photo detector is dependent on the dV/dt or the rate change in the voltage between the LED side and the photo detector. The spurious detector signals produced by displacement currents can therefore be disruptive to the normal communication process of the opto-coupler. Thus, there is a strong need to keep displacement current out of the photo detector, especially if the light signal is weak as is the case with silicon based LEDs.
Photo detectors associated with silicon integrated circuits are typically realized using semiconductor junctions. The junction based photo detectors include PN diodes, bipolar transistors, SCRs, and Triacs.
In general terms, the displacement current is given by       I          d      ⁢                           ⁢      i      ⁢                           ⁢      sp      ⁢                           ⁢      l      ⁢                           ⁢      a      ⁢                           ⁢      c      ⁢                           ⁢      e      ⁢                           ⁢      m      ⁢                           ⁢      e      ⁢                           ⁢      n      ⁢                           ⁢      t        =            C              c        ⁢                                   ⁢        o        ⁢                                   ⁢        u        ⁢                                   ⁢        p        ⁢                                   ⁢        l        ⁢                                   ⁢        i        ⁢                                   ⁢        n        ⁢                                   ⁢        g              ⁢                  ⅆ        V                    ⅆ        t            where Idisplacement is the displacement current or current flowing into the capacitance, Ccoupling is the capacitance between two electrically isolated conductors, and dV/dt is the rate change in voltage between the two isolated conductors.
The relative magnitude of this undesirable current can be estimated. Assume that the transparent insulator of an opto-coupler has a thickness of 300 μm and a relative dielectric constant of 2.8. The parallel plate capacitance per unit area is       C          i      ⁢                           ⁢      n      ⁢                           ⁢      s        =            ɛ              i        ⁢                                   ⁢        n        ⁢                                   ⁢        s                    t              i        ⁢                                   ⁢        n        ⁢                                   ⁢        s            where Cins=capacitance per unit area, εins=permativity of insulator, and tins=thickness of insulator. For tins=0.03 cm (300 μm) and εins=2.8×8.854c−14 then Cins=8.26 pF/cm2. Since a reasonable radius for a photo detector is 150 μm assume that the detector area is equal to π×0.0152 or 0.000707 cm2. Then Ccoupling=5.84 fF. To make the calculation worst case, assume that the fringe field coupling is 50% of the parallel plate capacitance for a total capacitance of 1.5×5.84 or 8.75 fF.
Assuming a transient voltage between the chip with the LED and the light detector of 106 V/sec, then icoupling=8.75 nA. Some opto-coupler specifications show a “common mode” slew rate as high as 109 V/sec, which produces a displacement current of 8.75 μA for a coupling capacitance of only 8.75F.
For an opto-coupler using a silicon junction avalanche LED, a reasonable quantum efficiency is 10−5. Assuming a detector quantum efficiency of 0.8, then for an LED current of 10 mA the photo current would be 80 nA. Although an 80 nA data signal current by itself can be readily detected by an amplifier circuit, a superimposed spurious displacement current 100 times greater in magnitude can make data extraction difficult and error prone.
FIG. 1 shows an example of the cross section of a silicon based opto-coupler. Two integrated circuits, 106 and 107, are shown separated by a transparent insulator 108. An LED 111 is built into integrated circuit 106 and emits light through the transparent insulator 108 to a light detector 112 of the receiving integrated circuit 107. Bond wire 104 connects package lead 102 to integrated circuit 106 and bond wire 105 connects package lead 103 to integrated circuit 107. Also shown is the lead frame die attach plate 110 for integrated circuit 106 and the lead frame die attach plate 109 for integrated circuit 107. Package lead 102 connects to integrated circuit 106 and establishes the base potential of integrated circuit 106 and package lead 103 connects to integrated circuit 107 and establishes the base potential of integrated circuit 107. The base potential of each integrated circuit is established through a power supply connection to the substrate of each integrated circuit. The surface of each integrated circuit may contain areas were the voltage is different from the base or substrate potential by several volts.
Under normal operation, light 112 is emitted from LED 111 to light detector 112. However, as shown in FIG. 1, a large electrical spike can exist between the base potentials of integrated circuits 106 and 107. The rate change in the voltage difference produces a displacement current 113 in the insulator 108 between integrated circuits 106 and 107. Some of the displacement current 113 flows out of the light detector 112 potentially disrupting operation.
As can be appreciated by one normally skilled in the art, LED 111 could also be a discrete GaAsP LED.